Vaporization of liquid fuels (e.g., hydrocarbons, alcohols) typically is achieved by indirectly supplying heat into a stream of liquid fuel via heat exchange with a hot wall. One disadvantage of this method is that the rate of vaporization is limited by the rate of heat transfer such that a relatively large surface area is required for fuel vaporization. Another disadvantage of this method, especially for vaporizing long chain hydrocarbons, is that heating the fuel stream to a vaporization temperature tends to cause fuel decomposition and formation of deposits. More specifically, coke formation is problematic. Moreover, preventing deposits from forming within fuel passages in a nozzle used for fuel delivery to a reformer is challenging.
Another known method for gasification of a fuel stream comprises mixing atomized fuel with a hot gas such as superheated steam that supplies the heat required for fuel vaporization and prevents coke formation. However, large amounts of superheated steam required in this method result in a large heat load for steam production.
Spray methods for atomization of liquid fuels, as known in the art, include air-blast or pressure atomizers, ultrasonic and electrospray atomizers. These spray systems are capable of providing a uniform distribution of atomized fuel across the entrance of a catalyst bed in a reformer. Such atomizers may include a co-flow of air that allows mixing of the fuel and oxidizer. However, very fine and uniform droplet size along with homogeneous fuel-air distribution, required to avoid coke formation and obtain temperature/mixture uniformity in the reformer, is difficult to achieve in practical reforming systems.
Ignition devices, such as metal spark or glow plugs, are widely used to ignite fuel-oxidizer mixtures at startup. By virtue of their location required for ignition, these devices are often subject to failure due to the reformer's high operating temperatures.
Monoliths are commonly used as catalyst substrates for the gasification of liquid fuel. Inhomogeneities in a fuel-oxidizer mixture are usually detrimental to monolith substrates, because inhomogeneities can lead to localized lean or rich zones causing hot spots or carbon precipitation regions, respectively. Little opportunity exists for these zones to re-mix within the monolith, because monolith substrates generally have long separated channels. Thus, monolith substrates are particularly vulnerable. In addition, carbon precipitation is favored in monoliths due to boundary layers that develop in these substrates on contact with the fuel oxidizer mixture.
Vaporization of liquid fuels poses significant problems, especially for fuels with high aromatic content and wide boiling point distribution. This can be attributed to the propensity of the heavier aromatic compounds in the fuel to form deposits or coke when vaporized at high temperatures. Accordingly, there is a need for a reforming or pre-reforming reactor capable of operating with a range of liquid fuels.
U.S. Pat. No. 4,381,187 to Sederquist (the “'187 Patent”) discloses a method in which a partially pre-vaporized fuel stream, mixed with air at an overall equivalence ratio greater than 3, is passed through a monolith catalytic structure thereby achieving gasification of the fuel in the stream. The '187 Patent requires mixing the fuel stream with a heated air stream and partial vaporization of the fuel prior to its introduction into the catalyst bed. Air temperature specified for the method is between 580° C. and 660° C. At these temperatures, coking may occur. The method of the '187 Patent requires supplying external heat for pre-heating the air. The method of the '187 Patent also requires the catalyst to be in a shape having wall surfaces extending in a downstream direction defining a plurality of parallel cells, for example, a conventional monolith. This configuration results in a comparatively low conversion rate of the reactants to the desired products. Moreover, in the method of the '187 Patent, the catalyst is chosen such as to initiate and sustain complete combustion, namely oxidation of part of the fuel to CO2 and H2O releasing heat. The '187 Patent discloses at column 1, line 49, that “once in vaporous form, fuel may be catalytically partially oxidized and reformed in an autothermal catalytic reactor.” Therefore, a separate reactor is required if a H2-rich gas stream is desired.
U.S. Pat. No. 4,255,121 to Sugimoto (hereinafter the '121 Patent) discloses a fuel reformer wherein a liquid fuel, oxidizer, e.g. air, and water are fed through a nozzle and atomized into one end of a cylindrical chamber containing a catalyst in the form of solid metal fins. A heating element wrapped around an exterior wall of the chamber provides an external source of heat at the nozzle for vaporization of the liquid fuel. A partially oxidized reformate exits in axial flow from the opposite end of the cylindrical chamber, where the reformate is ignited in a flame combustion. Disadvantageously, this process requires a source of external heat at the nozzle to vaporize the liquid fuel; thus the fuel is prone to coking at the nozzle. Moreover, the solid finned metal catalyst is not designed for thorough mixing. Additionally, axial flow is not commensurate with satisfactory temperature control or maximization of hydrogen product.
U.S. Pat. No. 3,978,836 to Noguchi et al. (hereinafter the '836 Patent) discloses a mixture heating unit for an internal combustion engine for the purpose of atomizing and vaporizing a fuel, so as to facilitate uniformity of distribution of a fuel mixture and to minimize emission of unburned gases. The heating unit features a heating element comprising a porous wick. The wick is wetted with fuel, and a resulting combination of vaporized fuel and air is ignited by a glow plug or spark plug. The resulting combustion flame ignites and activates a downstream catalyst layer to burn the mixture, which flows into an intake manifold of the engine.
In view of the above, it is therefore an object of the current invention to provide a reformer or pre-reforming reactor for partially oxidizing and cracking heavy components of a liquid fuel. It also is an object of the current invention to provide a catalytic reactor for the gasification of liquid fuels that yields partial oxidation products, specifically, carbon monoxide (CO) and hydrogen (H2). It is a further object of the current invention to provide a method and apparatus whereby steam or atomized water and/or carbon dioxide (CO2) may be added to the fuel/air stream to adjust the amount of hydrogen in the product stream. It is also an object of the current invention to provide a catalyst substrate that facilitates mixing of the stream flowing therethrough. It also is a further object of the current invention to provide a method and apparatus whereby the liquid fuel is vaporized without pre-heating by external means the fuel and air feeds.
The dependence of fuel conversion on an oxygen-to-carbon ratio (O:C) is known to one skilled in the relevant art. For the purposes of this invention, the term “O:C ratio” is defined as the number of oxygen atoms in the oxidizer divided by the number of carbon atoms in the liquid fuel, as fed to the reformer or pre-reformer. Tests of a conventional gasifier comprising a catalytic reactor and an inlet for prevaporized, premixed fuel and air indicated a linear increase in fuel conversion with increasing air. With increased air, or a higher O:C ratio, the catalyst temperature increased and higher fuel conversion was achieved, though at the expense of higher heat release and higher catalyst temperatures. It thus is a further object of the current invention to provide a method whereby gasification of liquid fuels is achieved by employing a fuel-rich fuel air mixture with an O:C ratio more suitable for efficient fuel conversion.